In this work, isothermal microcalorimetry (IMC) was used to study the behaviour of mixed cultures of Pseudomonas aeruginosa with Staphylococcus aureus and Escherichia coli. The species were firstly studied as pure culture at 10(6) CFU/mL densities in nutrient broth (NB). The species were then introduced into NB to give an equivalent inoculum of 10(6) CFU/mL of each. Additionally, the inoculum density of one species was maintained at 10(6) CFU/mL and the other varied between 10(3)-10(5) CFU/mL. The IMC power-time curves were characteristic for the species in the medium. The total heat output (Q(t)), amplitude of first peak (P-1), time of registration of first peak (t(1)), final metabolic/maximum peak (P-fmax) and time of registration of the final metabolic/maximum peak (t(fmax)) were calculated from the power-time data. A mixed culture of P. aeruginosa and S. aureus at equal densities exhibited metabolism synonymous to P. aeruginosa alone. When the density of P. aeruginosa was decreased, S. aureus gradually recovered showing power-time profiles that demonstrated this. Mixed cultures of P. aeruginosa and E. coli at equal densities showed power-time profiles representative of both species.